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United States Patent |
5,698,667
|
Speaks
,   et al.
|
December 16, 1997
|
Pretreatment of wood particulates for removal of wood extractives
Abstract
A process for extracting volatile organic compounds and pitch from wood
particulates, thereby virtually eliminating the emission of volatile
organic compounds into the atmosphere during the processing of wood
particulates into commercially useful products, such as oriented
strandboard, particle board, chipboard veneers, and pulp and paper
products. The removal of pitch permits the production of pulps of higher
brightness, requiring less chemical bleaching agents. Moreover, removal of
pitch eliminates pitch scale formation in pulp mills and on pulp and paper
machines with resultant improved efficiencies and reduced use of pitch
treatment chemicals. In the extraction process, a solvent or blend of
solvents, leach wood extractives, including volatile organic compounds and
pitch, from the wood particulates to produce a miscella. The miscella is
separated from the leached wood particulates and solvent contained in the
miscella is recovered and recycled for reuse. The wood extractives of the
miscella may be sold as chemical feedstocks or used as a fuel. Any
volatile organic compounds released as vapors in wood processing
operations prior to the extraction step are collected, absorbed onto
activated carbon particulates, and recovered for sale or combustion.
Inventors:
|
Speaks; Jerry R. (Union, WA);
Campbell; Roger O. (Federal Way, WA);
Veal; Michael A. (Federal Way, WA)
|
Assignee:
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Weyerhaeuser Company (Tacoma, WA);
North Pacific Paper Corporation (Longview, WA)
|
Appl. No.:
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579475 |
Filed:
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December 27, 1995 |
Current U.S. Class: |
530/202; 162/74 |
Intern'l Class: |
C09F 001/02; D21C 003/20 |
Field of Search: |
530/202
162/74
|
References Cited
U.S. Patent Documents
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|
Foreign Patent Documents |
60-163959 | Aug., 1985 | JP.
| |
Other References
T. Chen et al., "Using solid-phase extraction to assess why aspen causes
more pitch problems than softwoods in kraft pulping", Wood and Pulping
Chemistry, vol. 78, No. 10, Oct. 1995, Tappi Journal, pp. 143-149.
J. Brandal et al., "The Influence of Extractives in Groundwood Pulp on
Fibre Bonding", Pulp and Paper Magazine of Canada, Oct., 1966, pp.
T-431-T-435.
"New Pulps `Explode` onto Scene", Pulp & Paper Canada, Jun. 1987, p. 40.
|
Primary Examiner: Nutter; Nathan M.
Attorney, Agent or Firm: Christensen O'Connor Johnson & Kindness PLLC
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A method of removing volatile organic compounds and pitch from wood
chips, the method comprising:
(a) contacting the wood chips with a solvent for volatile organic compounds
and pitch;
(b) extracting volatile organic compounds and pitch from the chips into the
solvent under mild conditions of temperature and pressure to produce
extracted wood chips;
(c) separating a miscella comprising solvent, volatile organic compounds,
and pitch from the extracted wood chips;
(d) recovering solvent from the extracted wood chips and the miscella; and
(e) recycling the recovered solvent for reuse in contacting with wood chips
to extract volatile organic compounds and pitch.
2. The method of claim 1, wherein the step of recovering solvent from
extracted wood chips comprises heating wood chips soaked with solvent to
vaporize solvent from the wood chips.
3. The method of claim 1, wherein the step of recovering solvent comprises
subjecting the wood chips to pressure to express residual solvent and
pitch from the chips.
4. The method of claim 1, wherein the step of recovering solvent from the
miscella comprises distilling the miscella to reclaim solvent and produce
a separate product comprising pitch.
5. The method of claim 1, wherein the contacting comprises contacting with
a solvent miscible in water.
6. The method of claim 1, wherein the solvent is selected from the group
consisting of trichloromethane, diethyl ether, methanol, ethanol,
propanol, acetone, methyl ethylketone, kerosene, and methyl
isobutylketone.
7. The method of claim 5, wherein the step of extracting is carried out at
ambient temperature and pressure.
8. A method of removing extractable components from wood particulates, the
method comprising:
(a) contacting the wood particulates with a solvent for wood extractable
components;
(b) leaching extractable wood components from the wood particulates into
the solvent under mild conditions of temperature and pressure to produce
leached wood particulates and a miscella comprising solvent and
extractable wood components;
(c) separating the leached wood particulates from the miscella;
(d) recovering a solvent component and wood extractable components from the
miscella; and
(e) recycling the recovered solvent component to the step of contacting
with wood particulates.
9. The method of claim 8, wherein the recovering of a solvent component and
wood extractives comprises distilling the miscella to produce separate
solvent and wood extractive distillation products.
10. The method of claim 9, wherein the recovering of the solvent component
comprises distilling to produce a first product comprising solvent and a
second product comprising volatile organic compounds; and de-emulsifying a
distillate to produce a third product comprising pitch.
11. The method of claim 8, wherein the leaching of wood extractables is
leaching under ambient temperature and pressure conditions in a continuous
countercurrent extractor.
12. The method of claim 8, wherein the contacting is with a solvent
miscible in water.
13. A continuous process for removing volatile organic compounds and pitch
from wood chips, the process comprising:
(a) immersing wood chips in a solvent effective for extracting volatile
organic compounds and pitch from the chips for a period of time sufficient
to remove from about 50 to about 100% of the volatile organic compounds,
and from about 40 to about 80% of the pitch, from the chips to produce
extracted chips;
(b) separating extracted chips from a miscella comprising solvent and,
volatile organic compounds and pitch; and
(c) processing the miscella to produce a recyclable solvent product, a
volatile organic compound product, and a pitch product.
14. The process of claim 13, wherein the immersing in a solvent comprises
immersing in a water-miscible solvent.
15. The process of claim 13, wherein the processing of the miscella
comprises distilling the miscella.
16. The process of claim 13, wherein the immersing is under ambient
conditions of temperature and pressure.
17. The process of claim 13, wherein the immersing in a solvent comprises
immersing in acetone.
18. The process of claim 13, wherein the immersing is at a solvent:wood
chip ratio of from about 6:1 to about 1:1.
19. The process of claim 13, wherein the immersing is at a solvent:wood
chip ratio of about 2:1.
20. The process of claim 13, wherein the processing of the miscella
comprises recovering at least about 95% of the solvent of the immersing
step in the recyclable solvent product.
21. The process of claim 13, wherein the solvent comprises a mixture of a
first solvent for unsaponifiable wood extractives and a second solvent for
saponifiable wood extractives.
22. A method of extracting volatile organic compounds and pitch from wood
particulates, the method comprising:
extracting the particulates with a solvent under mild conditions of
temperature and pressure without significant dissolution of lignin from
the particulates and without significant attack of cellulosic components
of the particulates to produce extracted wood particulates having
significantly reduced pitch content and substantially reduced volatile
organic compound content;
separating a miscella containing the solvent from the extracted wood
particulates; and
recovering the solvent from the miscella.
23. The method of claim 22, wherein the extracting comprises extracting to
reduce a naturally-occurring pitch content of the wood particulates by
about 40 to about 80%.
24. The method of claim 22, wherein the extracting comprises extracting to
reduce naturally-occurring volatile organic compound levels of the wood
particulates by from about 50 to about 100%.
25. The method of claim 22, wherein the extracting comprises extracting
with a mixture of solvents.
26. The method of claim 22, wherein the mild conditions of extracting
comprise a temperature in the range from about 20.degree. C. to about
130.degree. C. and a pressure in the range of about 15 to about 25 psi.
27. The method of claim 22, wherein the extracting comprises extracting
with a solvent:wood ratio in the range from about 4:1 to about 1:1.
28. The method of claim 22, wherein the extracting is with at least one
water-miscible solvent.
29. The method of claim 28, wherein the at least one solvent forms only a
minimal azeotrope with water.
30. The method of claim 22, wherein the extracting with a solvent comprises
extracting with acetone.
Description
FIELD OF THE INVENTION
The invention relates to a process for the removal of wood extractives from
wood particulates without significantly affecting the integrity of
cellulosic components of the wood or removing lignin. More particularly,
the process of the invention uses solvent extraction techniques to remove
volatile organic compounds, as well as higher molecular weight pitch
compounds, from wood particulates thereby facilitating the further
processing of the wood into composite boards, paper, and pulp products
while significantly reducing the release of potentially harmful byproducts
into the environment.
BACKGROUND OF THE INVENTION
As a preliminary matter, wood can be viewed as consisting of two major
components, carbohydrates and lignin. Other components constitute a minor
part of the wood and manifest as intercellular material, and extraneous
substances that are related to the growth of the cells of the tree. The
cell walls of the wood are composed of polysaccharides, the chief of which
is cellulose. Lignin, on the other hand, is an amorphous substance, partly
aromatic in nature, that has been called a "cementing material" or an
"encrusting substance." It is insoluble in water and in most common
organic solvents. It is also insoluble in acids, but undergoes
condensation reactions in the presence of strong mineral acids. Lignin is
partly soluble in alkaline solutions and is readily attacked and
solubilized by oxidizing agents.
The extraneous substances of wood are deposited as cells grow, or after
they reach maturity. Most of these substances are relatively simple
compounds, having a low molecular weight. These low molecular weight
substances include pectins, proteins, and like substances that are soluble
in water or neutral organic solvents. The extraneous substances also
include "wood extractives" that include pitch and volatile organic
compounds.
To produce boards (oriented strand board, particleboard, veneerboard)
composite wood products, and paper and pulp products, raw logs or wood
fibrous material must be reduced to wood chips, flakes or sawdust. These
wood particulates are then further processed, either by bonding together
with a suitable glue to make board products, or undergoing pulping and
forming processes to produce a variety of papers, paper boards and
absorbent products. However, the processing of logs into wood
particulates, and thence into finished products, poses several challenges.
Some of these arise from the nature of wood, namely, that it includes not
only cellulosic fibers and lignin but also "wood extractives," as
discussed above. These naturally-occurring wood extractives are found in
both resin canals within the structure of the wood, as well as within the
parenchyma cells of the wood. In general, the extractives may be divided
into a higher molecular weight, higher boiling point fraction, commonly
known as "pitch", and a lower molecular weight, lower boiling point
fraction that falls within the definition of "volatile organic compounds."
The United States Environmental Protection Agency (EPA) has determined
that volatile organic compounds (VOCs) pose an environmental hazard when
they are released into the atmosphere. These VOCs are defined in 40 CFR
Part 51(s) as "any compound of carbon, excluding carbon monoxide, carbon
dioxide, carbonic acid, metallic carbides or carbonates, and ammonium
carbonate, which participates in atmospheric photochemical reactions."
Typically, these are volatile, low molecular weight organic compounds. The
EPA has promulgated regulations limiting the quantity of VOCs that a
manufacturing facility may release into the atmosphere.
The release of VOCs into the atmosphere is a long-standing problem in the
pulp and paper industry. Since VOCs occur naturally in timber, the
processing of timber into wood particulates facilitates the migration or
diffusion of VOCs to chip surfaces from which the compounds vaporize into
the surrounding atmosphere. As a practical matter, since the industry
requires a large inventory of wood chips for processing into board
products and as feedstock in the pulp and paper processes, significant
amounts of VOCs are released into the atmosphere from wood chip storage
piles. Further, as illustrated in FIG. 1, VOCs are also released into the
atmosphere during the processing of the wood chips into wood pulp
products. As shown, logs 5' are processed into chips in chip mill 10'
releasing VOCs 2' to the atmosphere. In pulping process operations, the
chips are stored in mounds 7' as inventory for the process. These mounds
continue to release VOCs 4' to the atmosphere. Some species of wood
produce more VOCs than others. For example, loblolly pine is higher in VOC
content than hemlock, and Douglas fir is intermediate between these two.
The VOC-containing chips are then processed in a pulp mill 12', either a
mechanical, thermomechanical, semi-chemical, or a chemical pulp mill, to
produce cellulosic and fibrous pulps. During this pulping process,
cellulosic fibers of the wood are separated from each other thereby
allowing entrapped VOCs to diffuse to fiber surfaces and vaporize into the
surrounding atmosphere. The cellulosic pulp produced may be bleached, and
is then formed into a continuous web and dried on a pulp drier or paper
machine 14'. During these processes, a further significant amount of VOCs
may be released into the atmosphere. The combined chipping, crushing,
pulping, and paper or absorbent product making processes release about
one-third of the total natural extractives in the wood into the atmosphere
(shown by arrows 2', 4', 6', and 8') as VOCs, and another one-third into
effluent water (arrows 20', 22' and 24'). The papermill product 15', such
as newsprint, writing paper, or absorbent products, includes the residual
of about one-third of the total amount of extractives, mainly pitch with
low amounts of VOCs.
As illustrated, wood particulates are also used as a raw material in
composite wood boardmaking processes. The logs are usually debarked and
reduced to flakes, fibers or other particulates on site then stored in
bins as inventory for the boardmaking. Before being consolidated into
boards, under heat and pressure, the wood particulates are dried to a
desired moisture content in ovens. VOCs are emitted into the environment
from the drying ovens and also from presses used to consolidate the dried
particulates, with a binder, into boards. Thus, a board manufacturing
process 26' also emits VOCs 28' while making boards 30'.
While the percentage of VOCs released into the atmosphere may appear small,
relative to wood particulate mass, the actual quantity is nevertheless
very significant. For example, a facility may process about 1,000-6,000
tons of wood chips per day. A 6,000 ton per day facility could produce 120
tons per day of VOCs. The EPA proposes limiting the amount of VOCs that
any wood chip processing facility releases into the atmosphere by
regulations requiting permits. Since a wood chip processing facility
represents a significant capital investment, operators must take steps to
limit VOC emissions while at the same time ensuring that processing
equipment operate at or near full capacity for an adequate return on
investment. To date, methods for limiting the quantity of VOC emissions
have focused on enclosing the atmosphere surrounding any wood chip process
that may release VOCs and subjecting air within the enclosure to treatment
for the removal of VOCs, before release of the air into the environment.
These methods require expensive equipment including large hoods to enclose
equipment, fans and ducts for transporting air containing VOCs, and
incinerators for combusting VOCs in the air. The methods also have high
combustion fuel costs.
The higher boiling portion of the wood extractives, the pitch, presents
separate and different problems in the processes for treating wood chips
to produce boards or mechanical and thermomechanical paper and pulp
products. In the pulp mill, the pitch separates from the cellulosic fibers
and gradually builds up a scale within the process equipment and ducting
of the mill. Ultimately, the pulp mill must be shut down so that this
pitch scale may be manually removed. To reduce the frequency of shut-downs
to remove pitch scale, sodium aluminate and alum is added to the pulping
process in an attempt to fix the pitch to the surface of the cellulosic
fiber. While this alleviates the equipment fouling problem, it does not
eliminate the problem. Indeed, the addition of aluminum chemicals also
poses a waste disposal problem since these chemicals are present in the
process water. Although this water is recycled, a portion must be treated
and disposed of. Pitch control requires additional operating costs for
treatment chemicals, labor and facilities, and disposal.
Pitch also causes significant equipment fouling problems in pulp dryers and
papermaking machines. In these capital intensive high speed machines, the
pulp is formed into continuous sheets on high speed belts, dewatered, and
dried. During these processes, colloidal pitch and pitch adhering to the
fibers is transferred to the rolls and machine "clothing" of the pulp or
papermaking machines to form a tacky, gummy surface deposit. This
ultimately results in reduced product quality and machine efficiency.
Removing the gum can require shutting down the papermaking machine,
chemical cleaning or removing the clothing, and cleaning all affected
surfaces. This results not only in cleaning costs and paper wastage
losses, but also in significant machine downtime with consequent economic
loss. Other methods of treatment include the use of continuous cleaning
chemicals and equipment. Some of these chemicals may contribute to the
release of VOCs and compositions with high biological oxygen demand (BOD)
and/or high chemical oxygen demand (COD) into the environment.
There exists a need to reduce or eliminate the release into the environment
of volatile organic compounds from processing operations that convert logs
into wood particulates and that convert the particulates into other useful
products. Further, there also exists a need to reduce or eliminate the
downtime of wood pulping facilities and papermaking machines caused by
fouling of equipment by pitch that occurs naturally in wood.
SUMMARY OF THE INVENTION
The invention provides a process for removing volatile organic compounds
and pitch from wood particulates. As a result, the invention substantially
reduces the emission of volatile organic compounds from board making
processes, chip pulping, and pulp and paper forming and drying processes.
The process of the invention also substantially reduces the mount of pitch
in wood particulates, thereby reducing or substantially eliminating pitch
fouling of equipment in pulp processing and papermaking processes.
Further, the process of the invention allows the production of a paper
pulp of superior strength, brightness, and optical properties.
According to the invention, wood particulates are contacted with a solvent
for the removal of wood extractives including VOCs and pitch. The solvent
extracts a proportion of naturally-occurring VOCs and pitch from the
particulates, and is separated as a "miscella" from the leached wood
particulates. The miscella, including solvent, water, VOCs, and pitch, is
subjected to a separation process that reclaims the solvent for reuse, and
produces a VOC product and a pitch product, which may be sold as a
chemical feedstock or used as a fuel. The leached wood particulates,
containing solvent, are subjected to a compression stage to express
residual solvent. Optionally, or in combination, heat may be applied to
vaporize and remove residual solvent. The vaporized solvent is condensed
and recycled with expressed solvent for reuse in the extraction process.
The leached wood particulates, now having substantially reduced VOCs and
pitch concentrations, may then be subjected to processes for the
production of composite board products or pulp products or absorbent
products or paper products, with significantly reduced emissions of VOCs.
The process of the invention removes from about 50 to about 100 wt % of the
VOCs present in the raw wood particulates. Further, the process also
removes from about 40 to about 80 wt % of the pitch. In certain
embodiments of the invention it may be preferred to use a mixture of
solvents, one solvent that is highly effective for the removal of VOCs,
and another solvent that is highly effective for the removal of higher
molecular weight pitch products. Alternatively, the wood particulates may
be subjected to a two-stage treatment process: One stage using a solvent
to remove saponifiable extractives (also known as "hydrophilic"
extractives), and another stage using a second solvent to remove
unsaponifiable extractives (also known as "hydrophobic" extractives).
The invention solves a long-standing environmental problem by reducing the
amount of VOCs released into the atmosphere in processes for converting
wood into useful products such as particle board, oriented strand board,
paper, absorbent products, and the like. Also, by removing pitch from the
wood particulates, the invention permits the realization of significant
cost savings in pulp mills and papermaking machine operations. The process
of the invention also allows a substantial reduction in pitch scale
formation in pulp mills, and on pulp and paper machines. This results in
significant improvements in mill efficiencies and reduced use of pitch
treatment chemicals, in pulp processes and process water, that pose a
disposal problem. Further, the removal of pitch from wood particulates
provides brighter wood particulates that resist age-darkening. This allows
the production of wood-containing pulp (also known as "mechanical pulp")
of higher brightness, thereby reducing the demand for chemical bleaches.
Additionally, the BOD and COD of process water are reduced, alleviating
the need for post environmental treatment.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes better
understood by reference to the following detailed description, when taken
in conjunction with the accompanying simplified process flow-type
drawings, not to scale, showing important process aspects of the invention
and the prior art wherein:
FIG. 1 is a schematic block flow diagram of wood chip processing showing
VOCs emissions in a prior art papermaking process and a prior art
chipboard process; and
FIG. 2 is a schematic flow diagram of an embodiment of the process of the
invention for VOC and pitch removal from wood chips;
FIG. 2A is a schematic flow diagram of an embodiment of a VOC-solvent-water
separation process of the invention;
FIG. 2B is a schematic flow diagram of another embodiment of
VOC-solvent-water separation process of the invention;
FIG. 3A is a schematic diagram of an embodiment of a chip extractor of the
invention;
FIG. 3B is a schematic diagram of another embodiment of a chip extractor of
the invention;
FIG. 3C is a schematic diagram of another embodiment of a chip extractor of
the invention; and
FIG. 3D is a schematic diagram of another embodiment of a chip extractor of
the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The continuous process of the invention uses an extractive solvent, that is
either a single liquid chemical compound or a mixture of such compounds,
for dissolving and removing naturally occurring wood extractives from wood
particulates suitable for use as chargestock in pulp and paper operations
or board manufacture. The term "wood particulates" refers to wood chips,
sawdust, flakes, shavings, and other such solid wood in particulate form.
It should be understood, that although the following description may refer
to "wood chips" the process of the invention is equally applicable to
other wood particulates.
The term "wood extractives," as used in the specification and claims,
refers to VOCs and pitch, and is measured as the extractives removed from
wood using an ether soxhlet extraction in accordance with TAPPI Standard
Test No. T204 om88 (modified to use diethyl ether as the extraction
solvent). This test does not distinguish between VOCs and pitch but
measures both as ether extractables of the wood. The percent wood
extractives removed by the extraction process of the invention is arrived
at by measuring the difference between the ether wood extractables in
samples of the wood particulates before and after undergoing the
extraction process of the invention.
While the specification and claims refer to VOCs and pitch as separate
components of wood extractives, it is recognized that in prior art
processes, not using the technology of the invention, emissions into the
environment include both VOCs and pitch. Under process conditions, a
proportion of non-VOC components also volatilizes and accompanies the VOCs
as an emission from the process. Frequently, these volatilized wood
extractives subsequently condense on process equipment, resulting in
fouling. According to the present invention, VOCs and volatilized wood
extractives are removed by extraction from the wood particulates.
The percentage of VOCs extracted from wood particulates is estimated by
subjecting the extracted wood particulates to an oven heating procedure at
105.degree. C. for 24 hours. The weight loss of wood particulates during
this procedure corresponds to the residual VOCs remaining in the extracted
particulates. Similarly, the quantity of VOCs in the raw particulates,
before extraction, may be estimated by heating the particulates to
105.degree. C. for 24 hours. Thus, the proportion of VOCs extracted may be
readily estimated from the measured amounts of VOCs present in the
particulates before and after extraction. The amount of pitch present
before and after extraction may be found by the difference, since the
total amount of wood extractives is determined by the TAPPI method, as
explained above.
The term "significantly reduced pitch content" with reference to extracted
wood particulates, means that at least about 40% of the
naturally-occurring pitch has been extracted from the particulates.
Preferably, from about 40% to about 80%, and more preferably from about
45% to about 75%, of the pitch is extracted.
The term "substantially reduced VOC content" referring to extracted wood
particulates, means that at least about 40% of naturally occurring VOCs
have been removed by extraction, preferably from about 50% to about 100%,
most preferably from about 75% to about 95%.
Preferably, the solvent used in the extraction process of the invention is
of a type that can be recycled for reuse in the process. To minimize
solvent recovery costs when distillation is used in the recovery process,
and to maintain the efficiency of the extraction process, it is preferred
that the extractive solvent is miscible with water under process
conditions and either does not form an azeotrope with water, or forms only
a minimal azeotrope. In preferred embodiments, the solvent is applied to
raw wood particulates that have not undergone a drying treatment to remove
water, and consequently commingles with water. This process is preferred
since it avoids costly drying processes. For ease of extraction, the
extractive solvent should have a high affinity for wood, i.e., the solvent
should readily diffuse or enter into spaces between cellulosic fibers to
leach out wood extractives. To facilitate recovery and reuse of the
solvent, the solvent should preferably have a physical property that
allows ready separation from water, for example, a preferred solvent boils
in the temperature range from about 40.degree. to about 75.degree. C.
under atmospheric pressure conditions, to facilitate separation by
distillation using steam as a heating medium. Alternatively, the solvent
could boil at a temperature higher than water, although this is
undesirable from an energy usage standpoint. Moreover, the solvent could
be immiscible with water, as long as it is able to leach out VOCs or
pitch, or both from wood particulates.
As indicated above, the extractive solvent may include a mixture of
solvents. In particular, the mixture may include a first solvent that has
a particularly high affinity for saponifiable components ("hydrophilic")
of the extractives, and a second solvent that has a high affinity for the
unsaponifiable ("hydrophobic") components. As a further alterative,
according to the invention, the wood particulates may be sequentially
subjected to one extractive process using a solvent for the removal of
saponifiable components, and another extractive process using a different
solvent for the removal of unsaponifiable components. The order of these
two extraction processes is not important.
Preferably, the extraction process is carried out under as mild conditions
of temperature and pressure as would require an extraction time of from
about two hours to about 10 minutes, or less, to minimize equipment size
for a particular rate of chips treated, in tons per hour. Most preferably,
the time of extraction is about 30 minutes to about one hour for
economical extraction equipment sizing. Extraction time, and hence size of
equipment, is also solvent dependent. Certain solvents remove extractives
at a faster rate and their leaching or solvent capability is not as
strongly adversely affected by increasing concentrations of extractives in
the solvent. Such solvents potentially minimize solvent recovery costs
because of their faster extraction rates requiting smaller volumes of
solvent.
Preferably, the mass ratio of solvent to wood particulates is in the range
of from about 6:1 to about 1:1, more preferably about 4:1 to about 1:1,
most preferably about 2:1. However, solvent:wood ratio also depends on
extraction time and temperature and pressure conditions. Thus, longer
extraction times allow a lower solvent:wood ratio for the same degree of
extraction for a particular solvent. Also, higher temperatures and
pressures allow reduced extraction time and reduced solvent:wood ratios.
The mass ratio of solvent to wood is measured as the total mass of solvent
that a particular mass of wood will encounter in a typical extractor.
Thus, even if the extractor is charged with "dirty" solvent that is
recycled, without first removing all wood extractives and water, the
solvent:mass ratio refers to the sum of the mass of pure make-up solvent
and the mass of solvent in the dirty recycled solvent stream, relative to
the mass of dry wood in the extractor.
Temperature and pressure conditions also impose constraints on the
selection of the solvent or solvents. Those solvents that are able to
effectively remove wood extractives from wood particulates, under mild
conditions of temperature and pressure, i.e., conditions that do not cause
significant dissolution of lignin or significant attack of wood cellulosic
components, are useful. Thus, it is preferred, within the equipment
economic size constraint mentioned above, that the extraction process
operate at a temperature in the range of from about 10.degree. to about
150.degree. C., more preferably from about 20.degree. to about 130.degree.
C. Preferred pressure conditions range from about atmospheric pressure
(14.7 psi) to about 50 psi, most preferably from about 15 to about 25 psi.
Again, the combination of temperature, pressure and time of extraction
should be selected to remove wood extractives without significantly
affecting yield, as discussed above.
According to the invention, the preferred solvent for the extraction of
VOCs is methylene chloride, 1,1,1-trichloroethane,
1,1,2-trichloro-1,2,2-trifluoroethane, trichlorofluoromethane,
dichlorodifluoromethane, chlorodifluoromethane, trifluoromethane,
1,2-dichloro 1,1,2,2-tetrafluoroethane, chloropentafluoroethane,
1,1,1-trifluoro 2,2-dichloroethane, 1,1,1,2-tetrafluoroethane,
1,1-dichloro 1-fluoroethane, 1-chloro 1,1-difluoroethane,
2-chloro-1,1,1,2-tetrafluoroethane, pentafluoroethane, tetrafluoroethane,
trifluoroethane, difluoroethane, parachorobenzotrifluoride, cyclic,
branched, or linear completely-methylated siloxanes, acetone, methyl
ethylketone, methyl isobutylketone, trichloromethane, ethyl ether, diethyl
ether, methanol, ethanol, pyridines, hexanes, benzene, and the like. Other
solvents may also be useful. Acetone is the most preferred solvent since
it is miscible with water, forms a minimal azeotrope with water, boils at
about 55.degree. C., and has a high affinity for wood, while also being an
excellent solvent for VOCs. In a preferred embodiment, wood particulates
are extracted by the method of the invention without predrying of the
particulates. In this embodiment, a polar solvent or mixture of solvents
or a hydrophilic solvent is preferred.
In accordance with the invention, solvents for the extraction of pitch are
also exemplified by the group described above. However, since pitch is of
higher molecular weight, these higher molecular weight extractives are
best extracted with a less polar solvent or solvent mixture. Preferably,
the solvent or solvent mixture is hydrophobic in nature, for example,
kerosene, cyclic saturated alkanes, such as hexane, octane, and the like.
Aromatic solvents, such as benzene, xylene and toluene, are also useful,
but temperature and pressure conditions should be controlled to avoid
significant dissolution of lignin. Such solvents are best employed after
the wood has been dried to remove water that may interfere with
extraction. Most preferably, however, the solvent is acetone, in which
case the wood does not have to be dried and a single solvent may be used
for the extraction of both VOCs and pitch. This also facilitates recovery
of the solvent by eliminating any requirement for duplication of solvent
recovery apparatus. Acetone also provides ease of separability from water,
low boiling point, relatively low cost, low toxicity and a favorable
environmental classification.
For ease of understanding the process of the invention, an embodiment of
the invention is illustrated in FIG. 2. As shown, raw logs 50 are charged
to a chipper 52 and then an optional chip crusher 53 for increase in
internal surface area. In prior art processes, during the chipping, chip
crushing and storage stages, VOCs are released and emitted into the
surrounding environment. As explained above, the EPA has set stringent
standards on the amount of VOCs that may be emitted. The chipping and chip
crushing processes may, therefore, optionally be enclosed within
substantially airtight, enclosed equipment from which air containing VOCs
is continuously removed, through ducts under an induced draft. This
VOC-containing stream may then be purified by passage through air
scrubbers and then optionally activated charcoal filters, or through
activated charcoal filters only.
Following the processing of solid product, the wood chips produced in
crusher 53, are charged to an extraction operation 56 that removes pitch
and VOCs from the wood chips. Preferably, this process is carried out in a
counter-current operation. By "countercurrent" it is meant that the
freshest solvent entering the extractor contacts chips that have already
flowed through most of the extractor volume, and fresh chips entering the
extractor first contact solvent that has already flowed through most of
the extractor. Ideally, in this type of flow arrangement, influent solvent
containing the lowest concentration of extractable material, contacts
chips from which a proportion of the extractives have already been
removed, so that the highest driving force for extraction is maintained.
This driving force is proportional to the difference between the
concentration of extractives in the solvent and the concentration of
extractives in the wood chips.
In the wood chip extractor shown in FIG. 3A, the extractor has a
cylindrical housing 300, preferably having a length-to-diameter ratio of
about 4:1. Wood chips enter the compression screw feeder 302 that includes
a progressively tapering screw thread within a sleeve. Thus, as the screw
thread conveys the chips toward the extractor, the chips are progressively
compressed in the tapering sleeve. This type of feeder is favored because
it can express some water from the chips, facilitating subsequent solvent
recovery. Any water expressed in the screw feeder is drained and removed
in conduit 303 and routed to VOC, pitch and solvent recovery processes.
The compressed chips enter the extractor near its top and flow downward
under gravitational force, and the mass of chips continuously added to the
extractor. The base of the extractor is supplied with a plurality of screw
feeders 304 aligned with the longitudinal axes parallel to the base of the
extractor. As these screw feeders 302 rotate about the axes, they convey
the chips towards the outlet compression screw feeder 306. During
compression of the chips in this outlet screw feeder, residual solvent is
removed from the chips. This solvent drains into conduit 307 and is routed
to a used solvent storage tank 308.
In order to remove wood extractives from the chips, solvent is added in at
least two points in the extractor. In order to mimic, as closely as
possible, countercurrent flow conditions, fresh solvent is injected near
the base of the extractor; and "dirty" solvent that has already passed
through the extractor, and that contains water and wood extractives, is
injected nearer the middle or upper section of the extractor. Thus, dirty
solvent is controlledly pumped from the used solvent storage tank 308
through outer concentric conduit 310 into the extractor at a location
about midway along the length of the extractor. Flesh solvent is injected
in an inner concentric conduit 312 that terminates near the base of the
extractor. Thus, as fresh solvent rises in the extractor, moving toward
the exit pipe 314, it encounters chips that have already undergone
extraction with dirty solvent. Consequently, the chips with the lowest
concentration of wood extractives come into contact with solvent having
the lowest concentration of wood extractives. This provides an optimum
driving force for further extraction of wood extractives from the chips.
In the upper part of the extractor, entering chips, containing naturally
occurring levels of wood extractives, first encounter dirty solvent. This
dirty solvent is still able to extract wood extractives from the chips
because of the high concentration of extractives present in the chips.
Ideally, flow of solvent in the extractor is of a plug-flow type. Thus,
there is little mixing between fresh and dirty solvent in the portion of
the extractor below the fresh solvent injection point. Under these
circumstances, the fresh solvent rises in the extractor as a "front" until
it meets with upwardly rising dirty solvent. At that point, commingling
takes place and the combined solvent mass, including extracted wood
extractives, rises upward through the extractor while leaching wood
extractives from chips, until the solvent exits the extractor in conduit
314 and is routed to used solvent storage 308. A portion of this solvent
is continuously removed and charged through conduit 60 to a solvent
reclamation process.
In an alternative embodiment of the extractor according to the invention,
shown in FIG. 3B, the extractor 320 has a cylindrical body inclined at an
angle of about 60.degree. to the horizontal. The extractor is supplied
with an internal screw 322 that has a longitudinal axis extending along
the central longitudinal axis of the extractor and that is coupled to a
drive motor 323. Threads of the screw extend outward from the root of the
screw at a screw pitch angle, toward the inner surface of the extractor
body 320, without touching the inner surface. Thus, the inclined screw 322
is free to rotate, under mechanical power, within the extractor. Chips are
fed into the solvent-filled extractor at an inlet near the extractor base
by means of a compression screw feeder 324. These chips are captured
between the helical threads of the rotating inclined screw of the
extractor and conveyed upward until they are expelled from the extractor
through a chip outlet 325 near the upper end of the extractor into an
outlet compression screw feeder 326. As explained before, the outlet
compression screw feeder compresses the chips and expresses residual
solvent from the chips. In order to achieve near countercurrent
conditions, acetone is injected into the inclined extractor through a
conduit 327 near the top of the extractor, and removed from the extractor
in an outlet conduit 328 near its base supplied with a chip filter 329.
In yet another embodiment of the chip extractor of the invention, shown in
FIG. 3C, the extractor 330 is inclined at an angle of about 60.degree.,
and is supplied with an internal pan conveyor 332. As is conventional, the
pan conveyor includes an endless belt extending substantially along the
central axis of the extractor. Containers, or "pans," for carrying chips
are formed along the belt by planar sheets, typically of metal, mounted
on, and extending at right angles from, the belt at spaced intervals. The
sheets extend toward, but do not touch the internal wall of the extractor.
Thus, chips are captured in the spaces between the plates and are carried
in the direction of movement of the belt. Chips are fed into the extractor
inlet 335 by a compression screw feeder 334, located near the top of the
extractor, on one side of the pan conveyor belt, and exit from the
extractor through an outlet 336 on the opposite side of the pan conveyor
belt, near the top of the extractor. The chips are carried away in a
compression screw feeder 337. Solvent enters into the extractor through a
conduit 338 near the outlet of the chips, and exits from the extractor
through a conduit 340 near the chip inlet 335. Thus, the flow through the
extractor is not completely countercurrent, but approximates
countercurrent conditions for at least the partially-extracted chips on
the exiting side of the pan conveyor.
In a further alternative embodiment of the chip extractor of the invention,
shown in FIG. 3D, the extractor 350 is cylindrical (with a horizontal
longitudinal axis) with a vee-shaped bottom to allow drainage of solvent.
Thus, chips enter through an inlet 352 near one end of the extractor, fed
by a rotary valve feeder 356. This type of feeder is an alternative that
may also be substituted for the screw feeders shown at the chip inlets of
the extractors of FIGS. 3A, B and C. The chips pour onto and are carried
by a centrally-mounted longitudinally-extending pan conveyor 358 toward
the opposite end of the extractor, while solvent is sprayed over the chips
from solvent distributor 362. The chips exit off the end of the conveyer
and fall into an exit chute 360. A compression screw feeder 364 then
removes the extracted chips for processing into pulp. The solvent is
removed through a conduit 366 that has a chip filter 365 and that is
located at the base of the extractor.
As can be seen from the above, the extraction of wood extractives from wood
chips may be achieved with a variety of extractor designs of the
invention. The nature of wood chips, and wood particulates, impose certain
limitations on the nature of the equipment. Wood chips, for example, tend
to interlock and form stable packed structures when placed within a
container, such as an extractor, or a silo. The above-described designs
overcome this tendency by providing either inclined screws, pan conveyors,
or screws near the base of the extractor to facilitate chip movement in
the extractor and chip removal from the extractor. The designs, especially
those of FIGS. 3B, 3C and 3D, also reduce channeling of wood chips from
inlet to outlet of the extractor and facilitate control of chip residence
time in the extractors.
In the extraction stage 58, the wood chips are immersed in the extraction
solvent supplied in conduit 148 from solvent storage 146. Mild agitation,
while preferred, is not necessary. During the immersion, solvent surrounds
and penetrates the wood chips dissolving and leaching wood extractives,
including VOCs and pitch, from the structure of the wood chip. Preferably,
the solvent penetrates to and removes extractives from the resin canals of
the wood as well as the parenchyma cells of the wood. This removal or
"leaching" of extractives from the wood takes place under conditions of
temperature and pressure that do not cause substantial attack of the
ligninor cellulosic component of the wood. Thus, the high temperatures and
pressures used in prior art processes designed to delignify wood or to
pulp wood using solvents (omen in combination with catalysts) are not
employed. Instead, the integrity of the cellulosic component is maintained
as wood extractives are leached out. Moreover, the lignin component of the
wood is also not affected, or only insignificantly affected, so that the
wood particulates are not pulped. Only removal of a sufficient proportion
of extractives to substantially reduce subsequent VOC release from the
leached wood chips and to reduce the need for pitch-scale treatment
chemicals in subsequent pulping operations, is required according to the
invention. In certain instances, external heat may be supplied to
facilitate leaching. Moreover, in certain instances, pressure may be
applied in the extraction process to prevent vaporization of the solvent.
However, in the preferred embodiment using acetone as a solvent, external
heat may not be needed, nor may pressure have to be applied. Thus, the
leaching or extraction can take place at ambient conditions of temperature
and at about atmospheric pressure.
The extracted wood chips are separated from solvent in the extractor(s) and
transported to optional chip pressing operations 62 for removal of
residual solvent and extractives, for instance in screw presses. The
solvent, containing water, pitch and VOCs, now called a "miscella" is
removed in conduit 60 for processing to recover solvent for reuse, and
pitch and VOCs for sale or combustion.
In the optional screw presses, the extracted wood chips are subjected to
mechanical pressure causing squeezing and compression of the chips. As a
result, residual solvent containing pitch is expressed from the chips.
This liquid is conveyed in a conduit 63 to the solvent and pitch recovery
processes, as will be described later. The compressed wood chips, still
containing residual solvent, are charged to a solvent removal stage 66.
Solvent removal may be effected by conventional means, such as charging to
a rotary drum dryer, or continuous dryers that comprise a multiplicity of
drying stages enclosed in a housing and subjected to direct contact steam
that removes solvent from a substrate to be dried. Solvent vapors removed
during this stage are carried by conduit 68 to processes for solvent
recovery. The substantially solvent-free leached chips, with reduced VOC
and pitch content, are charged to board making or pulping processes,
generally designated by the numeral 72. As a result of the extraction of
VOCs and pitch, in the process of the invention, VOC emissions during the
boardmaking or pulping operations are significantly reduced. Furthermore,
as explained above, paper and absorbent product manufacturing processes
are enhanced, by the virtual elimination of pitch that causes fouling of
equipment and related loss in efficiency and production. The quality of
paper and pulp products is also improved, as explained above. Further, if
the chips are used in boardmaking, then bonding strength is improved so
that board quality is enhanced while VOC emissions are substantially
reduced.
In an important aspect of the invention, the extractive solvent used in the
VOC and pitch extraction stage is recovered and recycled for reuse. As
shown in the illustrative embodiment of FIG. 2, liquid streams 60, 63 and
68 containing solvent, from extractor(s) 56, optional chip pressing 62,
and solvent removal 66, respectively, are gathered in header 70 which
charges the solvent-containing fluids to a first distillation column 72.
The distillation column preferably has three stages of separation, when
acetone is used as a solvent. Clearly, the number of stages will vary
depending upon physical properties of the extractive solvent used.
However, the distillation column may be readily designed with the aid of
commercially available multi-component distillation software, such as
ASPEN PLUS, supplied by Aspen Technology Inc. of Cambridge, Mass.
In the embodiment shown in FIG. 2, distillation column 72, preferably under
partial vacuum pressure, is supplied with steam 74 as a heating medium to
raise the liquid in the base of the distillation column to a temperature
at or above its bubble point. Under these conditions, vapors containing
acetone, VOCs and some water vapor, rise to the top of the distillation
column 72 and are removed in overhead conduit 80. These overhead vapors
are condensed in cooler-condenser 76, supplied with water at about
20.degree.-25.degree. C. (or cooler) as a cooling medium. The
cooler-condenser 76 may be of conventional shell and tube construction,
plate and frame construction, and the like. Condensate is removed from the
cooler-condenser in conduit 82 and is charged to a solvent, VOC and water
storage tank 100.
A bottom product stream 78 is also withdrawn from the first distillation
column 72. This bottom product stream contains a much lower proportion of
solvent than the charge supplied to the distillation column in conduit 70,
but yet contains some solvent, as well as water and pitch. In one
embodiment, substantially all of the VOCs are removed in the overhead
product from column 72. The substantially VOC-free bottom product is
charged to a second distillation column 84 for recovery of solvent. This
distillation column 84 is preferably also under partial vacuum, but a
greater vacuum than in the first column, is supplied with heat, preferably
through higher pressure steam than supplied to the first column, as shown
by arrow 88. As a result of the higher temperature at the base of the
distillation column and the increased vacuum, any remaining solvent is
stripped from the charge to the distillation column. Consequently, a
bottom product stream, substantially free of solvent and VOCs, is
withdrawn from the distillation column in conduit 90 and charged to
separator 120, as will be discussed later. An overhead product stream,
containing mainly solvent, some water, and any residual VOCs, is removed
from an overhead portion of the distillation column through conduit 86.
This vapor stream is condensed in cooler-condenser 92. As before, the
cooling medium in this cooler-condenser may be cooling water at about
20.degree.-25.degree. C., or colder. Condensate is carried from the
cooler-condenser in conduit 94 and charged to the solvent, VOC, and water
storage tank 100.
As explained above, the bottom product stream carried in conduit 90 from
the second distillation column 84 contains an insignificant amount of
residual solvent, in addition to pitch and water. This bottom product is
charged to separator 120, preferably a heated tank, where heat is supplied
by internal heating coils to raise the temperature of liquid to a
temperature that favors separation of pitch and water, with the aid of a
de-emulsifier, and that maintains the pitch in a pumpable viscosity range.
Pitch separates from the water and accumulates in a layer. This pitch
layer is then withdrawn in conduit 124 for potential sale. As an
alternative, the pitch may be burned as a fuel since it has a heating
value approximately 85% of that of No. 6 fuel oil. Mother product stream
126, containing mainly water, is also removed from the separator 120. This
water is suitable for reuse within the process, as process water, or may
be released to other mill uses or recycled back to 84 for further
separation.
Rectifier 130 receives charge from the solvent, VOC and water storage tank
100. Thus, rectifier 130 is essentially utilized to separate solvent and
VOCs from water, although minor quantities of pitch may also be present.
Preferably, rectifier 130 is supplied with steam 134 near its base as a
reboil heating medium. As a result of heating liquid in the base of
rectifier 130 to its bubble point or above, a bottom product substantially
free of VOCs and solvent is produced. This predominantly water-containing
product stream is removed in conduit 132, for use in other mill processes
or for separation in the separator 120, if it contains significant amounts
of residual pitch.
At the same time, the rectifier also produces an overhead product, rich in
solvent, that is removed in conduit 136 and charged to a cooler-condenser
140. In this cooler-condenser, the solvent is condensed and the condensate
is transported away in conduit 138 to dry solvent storage 146 for reuse in
the extraction process. A side drawoff stream from the rectifier 120,
containing mainly VOCs, is cooled in cooler 148 and the cooled liquid is
routed through conduit 144 to VOC storage tank 150. The stored VOCs are
routed to a combustion process 154 for disposal or to sales.
In an alternative, preferred embodiment, the VOCs are produced as two
separate products. With reference to FIG. 2A, the first distillation
column 72 produces an overhead product cooled in cooler condenser 76,
containing light VOCs (LVOCs) that is stored in LVOC, solvent, and water
storage tank 200. The second distillation column 84, produces an overhead
product condensed in cooler condenser 92 containing heavier VOCs (HVOCs),
and water. Consequently, instead of combining the overhead products by
charging both to a single solvent, VOC and water storage tank, the
overhead products are kept separate and are charged to separate storage
tanks. This allows the production of separate LVOC and HVOC products. In
order to produce the separate products, the mixture of LVOCs, solvent and
water from storage tank 200 is charged to a rectifier 210 for separation
into a bottom stream 218 containing mainly water is routed to reuse or
disposal. A middle drawstream 222 containing mainly solvent is condensed
in a condenser 220. The condensed solvent is routed to the dry solvent
storage tank 146, as in the process described in FIG. 2. Referring again
to FIG. 2A, an overhead LVOC product of the rectifier 210 flows through
conduit 212 to cooler-condenser 214. The condensed LVOC product is stored
in an LVOC storage tank 216.
The HVOC product is produced by charging the mixture in storage tank 202 to
a rectifier 224. In this rectifier, the mixture is separated into an
overhead product, containing mainly solvent, that is cooled and condensed
in a condenser 226 before being charged to dry solvent storage tank 146. A
mid-column drawoff stream, containing mainly HVOCs, is cooled in a cooler
228 and then routed to HVOC product storage tank 230. The rectifier bottom
product, carried in conduit 232, contains mainly water and pitch. This
mixture is routed in conduit 232 to a heated de-emulsifier tank 234 where
the pitch separates from the water. The pitch is removed in conduit 233,
for sale or use as fuel, while the water is routed in conduit 235 for use
in the process, or disposal.
Clearly, the process described in FIG. 2A can also be operated with a
single rectifier operating on two cycles. In one cycle, the rectifier is
used to separate the mixture from tank 200 into LVOCs, water and solvent.
In another cycle, the rectifier is used to separate the mixture from
storage tank 202 into HVOCs, solvent and water. Storage tank sizing and
distillation columns 72 and 84 overhead product volumes dictate the length
of each of the cycles.
In a further alternative more preferred embodiment, shown in FIG. 2B, the
rectifier 130 has an overhead product drawoff, two side product drawoff
streams, and a bottom product stream. The overhead stream is rich in
LVOCs; an upper near-top-column drawoff stream is rich in solvent; a lower
near mid-column draw off stream is rich in HVOCs; and the bottom stream is
substantially free of VOCs and solvent but contains pitch and water. This
clearly assumes that the boiling point of the selected solvent is
intermediate the LVOCs and the HVOCs. If not, then the drawoff
configuration may readily be altered to accomplish the separation.
Regardless, in the type of rectifier, pump arounds may have to be
installed in order to remove or add heat to the distillation column to
facilitate separation between the LVOCs, HVOCs, and solvent. The function
of these pumparounds is to controlledly modify the temperature profile of
the distillation column, thereby facilitating separation of LVOCs and
HVOCs and solvent. A person of ordinary skill in the art, having read this
disclosure, and having access to distillation column design software, such
as the software named above, would readily be able to design a rectifier
with appropriate pumparound volume and temperature to achieve the
separation.
It is important to note that the volatile organic compound product
produced, and the pitch product produced, are not necessarily "pure."
Rather, the VOC product may contain at least some, although minimal,
amount of solvent, as well as water. Preferably, the amount of solvent in
the VOC product is minimized to reduce the cost of adding makeup solvent
to the process. Nevertheless, at least some proportion of the solvent will
be lost in the VOC, and possibly pitch, products for economical
distillation operation.
The pitch product will contain pitch as well as water. Pitch by itself
solidifies at room temperature and is difficult to handle. While the pitch
may be spray-dried into pellets for handling, it is preferred that the
pitch product contain less than about 50 wt % solids so that it may be
maintained in a liquid state, either at ambient temperature or with the
addition of economically minor amounts of heat or waste heat. This liquid
pitch product is more readily pumped into heated tank cars for sale.
The process of the invention removes volatile organic compounds from wood
particulates thereby allowing processing of these wood particulates
without the release of VOCs into the environment. Moreover, the process of
the invention removes pitch from wood particulates thereby facilitating
further processing of the wood particulates into useful products. Further,
the invention provides two additional useful products, namely, VOCs and
pitch, that may be sold as byproducts or used as fuel, thereby enhancing
the economics of the process of the invention.
The following examples are illustrative of aspects of the invention and do
not in any way limit the scope of the invention, as described above and
claimed herebelow.
EXAMPLES
Example 1
Comparison of Solvents for the Removal of Wood Extractives
A series of solvents were tested to determine which was most effective for
the extraction of wood extractives, including volatile organic compounds
and pitch. In each of the tests, 50 gram batches of oven dried Lodgepole
Pine wood chips were extracted with solvent at a solvent:wood mass ratio
of 4:1. Samples of each batch were each analyzed for wood extractives,
using a modified TAPPI test method T204 om88 with diethyl ether as the
extraction solvent, before and after extraction with the test solvents.
In each case, the batch of wood chips was subjected to a batch extraction
process. The wood chips were not predried, so that their condition
approximated that of wood chips normally received for treatment in a wood
pulping facility, or used in a composite wood product manufacturing
facility. The wood chips were preheated with atmospheric steam for 30
minutes. During this time, the wood chip temperature rose to about
95.degree. C. The wood chip batch was then immediately submerged in the
extraction solvent. In each case, the solvent:wood ratio was 4.0 and the
extraction time was 30 minutes. After extraction, solvent was drained from
the chips, and the chips were subjected to a second heating cycle of 30
minutes with atmospheric steam. Thereafter, the chips were subjected to a
second extraction cycle using the same solvent at the same solvent:wood
ratio. After draining solvent from the chips, the chips were analyzed to
determine the amount of residual wood extractives. The percent wood
extractives removed was calculated for each batch and the results are
reported in the accompanying Table 1.
TABLE 1
______________________________________
Treatment Solvent
Percent Extraction
______________________________________
Peracetic Acid 45.8
Caro's Acid 14.2
Hypochlorous Acid
37.5
Deionized Water 41.0
Acetone/Water 80/20
54.4
Acetone 100% 65.0
______________________________________
These results indicate that acetone is the best solvent for the removal of
wood extractives from Lodgepole Pine. Acetone has advantages over the use
of an 80/20 acetone/water mixture, and is also superior to the other
solvents tested. It is theorized, without being bound, that oxidized acids
(or alkaline reagents), depend upon chemical reactions that convert wood
resins in order to achieve extraction. Not only is this from a
thermodynamic perspective not as effective as direct solution of the
extractives in an organic solvent, but alkaline extractions have several
disadvantages. These include the darkening of wood fibers which would
result in higher fiber bleaching costs. Moreover, the nonselective nature
of caustic treatments result in loss of yield. Also, caustic extracts are
extremely toxic and costly to treat.
Example 2
Process Conditions for the Removal of Wood Extractives
A series of acetone extractions were conducted to determine conditions
suited for the efficient removal of wood extractives. In each case, a 50
gram batch of oven dried wood chips was treated in a solvent:wood ratio of
4.0. The wood chip species evaluated were seven batches Ponderosa Pine
(PP) and four batches of Douglas Fir (DF) as well as a PP control batch.
During the extraction processes, steam preheating time, acetone extraction
time, and post-steaming times were varied. Steam was supplied at ambient
pressure, and the extractions were carried out at ambient temperatures and
pressures. In each case, the extracted wood chips were finally squeezed in
a press at 1500 psi for 5 minutes. A modified TAPPI test method, T204
om88, using diethyl ether as the extraction solvent, was used to determine
the percentage of wood extractives removed from the samples. The results
are shown in Table 2.
TABLE 2
__________________________________________________________________________
Steam #1 Extraction #1
Steam #2 Extraction #2
Press
Extraction
time, minutes
0 15 30 15 30 0 15 30 15 30 5 %
__________________________________________________________________________
PP1 X X 62.5
PP2 X X X 48.6
PP3 X X X 53.3
PP4 X X X 64.6
PP5 X X X X X 58.5
PP6 X X X 78.2
PP7 X X X X X 73.0
Control PP H.sub.2 O X 17.6
DF X 48.5
DF X X 53.6
DF X X X X 54.2
DF X X X X 57.4
__________________________________________________________________________
From the above table, presteaming with atmospheric steam did not appear to
enhance extraction. Indeed, presteaming appears to reduce extraction.
While multi-stage extractions show slight increases in overall extraction,
this increase may not justify the additional equipment required in a
commercial operation. Increasing the extraction time, in a single- or
multiple-stage extraction, is effective in increasing the percent wood
extractives removed.
Example 3
Variation of Percentage of Wood Extractives Removed with Extraction Time,
Using Acetone as a Solvent
A batch of Lodgepole Pine chips was sampled and tested as described in
TAPPI T204 om88, modified to use diethyl ether as a solvent, to ascertain
the amount of wood extractives in the chips. Then, samples of the chips
were each treated with acetone for 3, 5, 10, and 20 minutes, respectively.
Each extracted chip sample was then air dried, ground to 1 mm size
particulates, and extracted in the same modified TAPPI method to determine
residual wood extractives. The percent wood extractives removed was
calculated for each extracted sample and the results were tabulated in
Table 3.
TABLE 3
______________________________________
Time of Ether
Extraction Extractables
Extraction
(min) (wt. %) %
______________________________________
0 2.9 0
3 2.3 21
5 1.9 35
10 1.5 48
20 0.75 74
______________________________________
The results show that wood extractives were reduced from 2.9% in the raw
Lodgepole Pine chips to 0.75 wt. % in 20 minutes. This represents an
extraction of about 75% of the wood extractives. Moreover, after only 5
minutes, 35% of the wood extractives have been removed. Tests indicated
that volatile organic compounds were virtually completely removed from the
chips, even after only 5 minutes. Thus, longer extraction time are only
needed if it is desired to remove increasing quantities of pitch. It is
theorized, without being bound, that lower molecular weight wood
extractives are more soluble and are therefore extracted at a faster rate
than the higher molecular weight components. Consequently, VOCs are first
removed, followed by those wood extractives that are likely to become
volatilized under wood chip pulping conditions, and composite board making
conditions. Therefore, extraction need only proceed to remove these
components, unless higher molecular weight, less soluble pitch must also
be removed for other purposes.
Example 4
Comparison of Alternative Solvents for the Removal of Wood Extractives
A series of wood chip extractions were conducted with organic solvents to
determine their relative ability to leach extractives from wood. The
solvents include methanol, ethanol, 2-propanol, methyl iso-butyl ketone,
hexane, acetone, and water.
Samples of raw Lodgepole Pine wood chips were each extracted according to
TAPPI T204 om88, modified to use diethyl ether as a solvent, to determine
initial wood extractives content. In a first comparison, batches of wood
chips were each extracted with a specific solvent, at its boiling point,
for either 20 minutes or 4 hours, respectively. The extracted wood chips
were then air dried, ground to 1 mm size, and again extracted with diethyl
ether, in the modified TAPPI test method T204 om88, to determine residual
wood extractives.
A second set of wood chip samples were first air dried, then ground to 6 mm
particle size, before being extracted for 4 hours at the solvent boiling
point. Thereafter, the extracted wood particulates were ground to 1 mm
size, and extracted with diethyl ether, as above, to determine residual
wood extractives.
Finally, samples of wood meal were also extracted with each solvent for 4
hours at the solvent boiling point to determine the limit of wood
extractives removal achievable with the particular solvent. The percentage
of wood extractives removed in each extraction was calculated and the
results are tabulated in Table 4.
TABLE 4
______________________________________
% Extractives Removed
20 minute 4 hour 4 hour
Extraction Reflux Reflux Reflux
Conditions chips chips wood
Sample Type 6 mm 6 mm meal
______________________________________
Methanol 68 75 95
Ethanol 62 73 96
2-Propanol 66 75 94
Acetone 67 75 96
Methyl Isobutyl Ketone
41 70 96
Hexane NA 18 86
Water 21 17 38
______________________________________
As can be seen from the above table, the hydrophilic solvents appear to be
superior to the hydrophobic solvent, hexane, as an extraction solvent.
Moreover, percent extraction increases with time of extraction, although
the increase is small relative to the increase in time required. Methanol
and acetone appear to be the best solvents. However, methanol poses
toxicity issues.
Based on the percentage extraction achieved with wood meal, the practical
upper limit of wood extractive removal appears to be about 95%. However,
as explained before, virtually all volatile organic compounds will be
removed, and the residual wood extractives are expected to comprise only
the higher molecular weight, and specifically, more hydrophobic, wood
extractive components.
Example 5
Determination of the Effect of Wood Particle Size and Handling Conditions
on Removal of Wood Extractives
In order to test the effect of particle size, wood chips were treated in
equipment that would either (1) reduce average particle size or, (2) cause
fractures in the wood chips opening internal surfaces and reducing average
chip thickness. A batch of chips was treated with a Rader DynaYield Chip
Conditioner, designed to squeeze those wood chips that have a thickness
greater than 1.5 mm. In this conditioner, the greater the thickness of the
charged wood chip, the more work is applied to the wood causing
delamination along the wood grain. In effect, this reduces the apparent
particle thickness without significantly decreasing chip size or
integrity.
Another batch of chips was treated in a Prex screw press. This equipment
causes a larger size reduction. However, it is also known that the quality
of pulp produced from chips treated through a screw press, or like
equipment, such as the Sprout-Bauer Pressifine, French Oil Press, and Prex
screw is minimally affected.
A sample of the wood chips was extracted using TAPPI T204 om88 test method,
modified to use diethyl ether as a solvent, to determine the percent wood
extractives present. Those chip batches treated in the Rader Chip
Conditioner and the Prex screw feeder and a control batch were each
separately extracted with acetone, under the same conditions of
concentration, solvent:wood ratio, temperature and pressure. A sample of
the extracted chips was again analyzed by the TAPPI method to determine
residual wood extractives. The percentage of wood extractives removed was
calculated. The results are shown in Table 5.
TABLE 5
______________________________________
Wood Chip Size
Control Chip
Rader Conditioner
Prex Screw
______________________________________
Over Thick > 10 mm
60% 72% --
<10 mm 58% 78% 84%
<6 mm 65% 67% 88%
Pins 82% -- --
Fines 91% -- --
______________________________________
As shown in the table, treating chips in a Rader conditioner allows some
increase in the removal of wood extractives, especially for larger size
wood chips. This is to be expected, since fracturing the larger wood chips
allows better penetration of the solvent into the interior of the chip.
The effect of increased extraction is even greater with chips treated with
the Prex Screw equipment. Again, this is explained by the greater degree
of size reduction and fracturing of the chips that is achieved with this
equipment that facilitates penetration by the solvent into the chip and
removal of wood extractives.
While the preferred embodiment of the invention has been illustrated and
described, it will be appreciated that various changes can be made therein
without departing from the spirit and scope of the invention.
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